Rate selection for eigensteering in a mimo communication system

ABSTRACT

Techniques for selecting rates for data transmission on eigenmodes of a MIMO channel are described. An access point transmits an unsteered MIMO pilot via the downlink. A user terminal estimates the downlink channel quality based on the downlink unsteered MIMO pilot and transmits an unsteered MIMO pilot and feedback information via the uplink. The feedback information is indicative of the downlink channel quality. The access point estimates the uplink channel quality and obtains a channel response matrix based on the uplink unsteered MIMO pilot, decomposes the channel response matrix to obtain eigenvectors and channel gains for the eigenmodes of the downlink, and selects rates for the eigenmodes based on the estimated uplink channel quality, the channel gains for the eigenmodes, and the feedback information. The access point processes data based on the selected rates and transmits steered data and a steered MIMO pilot on the eigenmodes with the eigenvectors.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present application for patent is a Continuation of patentapplication Ser. No. 11/128,843 entitled “Rate Selection ForEigensteering In A Mimo Communication System” filed May 12, 2005, nowissued, and assigned to the assignee hereof and hereby expresslyincorporated by reference herein.

BACKGROUND

I. Field

The present invention relates generally to communication, and morespecifically to techniques for selecting rates for data transmission ina multiple-input multiple-output (MIMO) communication system.

II. Background

A MIMO system employs multiple (T) transmit antennas at a transmittingstation and multiple (R) receive antennas at a receiving station fordata transmission. A MIMO channel formed by the T transmit antennas andthe R receive antennas may be decomposed into S spatial channels, whereS≦min {T, R}. The S spatial channels may be used to transmit data inparallel to achieve higher throughput and/or redundantly to achievegreater reliability.

Each spatial channel may experience various deleterious channelconditions such as, e.g., fading, multipath, and interference effects.The S spatial channels may experience different channel conditions andmay achieve different signal-to-noise-and-interference ratios (SNRs).The SNR of each spatial channel determines its transmission capacity,which is typically quantified by a particular data rate that may bereliably transmitted on the spatial channel.

Rate selection refers to the process of selecting suitable rates fordata transmission, e.g., on the spatial channels of the MIMO channel. A“rate” may be associated with a particular data rate or information bitrate, a particular coding scheme or code rate, a particular modulationscheme, and so on to use for a data stream. For a time variant MIMOchannel, the channel conditions change over time and the SNR of eachspatial channel also changes over time. The different SNRs for differentspatial channels plus the time varying nature of the SNR for eachspatial channel make it challenging to select the proper rates for thespatial channels.

There is therefore a need in the art for techniques to select rates in aMIMO system.

SUMMARY

Techniques for selecting rates for data transmission on eigenmodes of aMIMO channel are described herein. An eigenmodc may be viewed as anorthogonal spatial channel obtained by decomposing a channel responsematrix for the MIMO channel. The techniques may be used for downlinkdata transmission from an access point (AP) to a user terminal (UT),uplink data transmission from the user terminal to the access point, andpeer-to-peer data transmission between two user terminals.

According to an embodiment of the invention, an apparatus is describedwhich includes a channel processor and a controller. The channelprocessor receives a pilot (e.g., an unsteered MIMO pilot) via a firstcommunication link (e.g., the uplink) and derives a channel estimate forthe first communication link. The controller receives feedbackinformation indicative of the channel quality of a second communicationlink (e.g., the downlink) and selects rates for eigenmodes of the secondcommunication link based on the feedback information and the channelestimate.

According to another embodiment, a method is provided in which a pilotis received via a first communication link. Feedback informationindicative of the channel quality of a second communication link is alsoreceived. Rates for eigenmodes of the second communication link areselected based on the feedback information and the pilot.

According to yet another embodiment, an apparatus is described whichincludes means for receiving a pilot via a first communication link,means for receiving feedback information indicative of the channelquality of a second communication link, and means for selecting ratesfor eigenmodes of the second communication link based on the feedbackinformation and the pilot.

According to yet another embodiment, a method is provided in which afirst unsteered MIMO pilot is transmitted via the downlink. A secondunsteered MIMO pilot and feedback information are received via theuplink. The feedback information is indicative of the downlink channelquality, which is estimated based on the first unsteered MIMO pilot.Rates for eigenmodes of the downlink are selected based on the feedbackinformation and the second unsteered MIMO pilot.

According to yet another embodiment, an apparatus is described whichincludes a pilot processor, a controller, and a spatial processor. Thepilot processor generates a pilot for transmission via a firstcommunication link. The controller sends feedback information indicativeof the channel quality of a second communication link. The spatialprocessor receives a data transmission on eigenmodes of the secondcommunication link. The data transmission is sent at rates selectedbased on the pilot and the feedback information.

According to yet another embodiment, a method is provided in which apilot is transmitted via a first communication link. Feedbackinformation indicative of the channel quality of a second communicationlink is also sent. A data transmission, which is sent at rates selectedbased on the pilot and the feedback information, is received oneigenmodes of the second communication link.

According to yet another embodiment, an apparatus is described whichincludes means for transmitting a pilot via a first communication link,means for sending feedback information indicative of the channel qualityof a second communication link, and means for receiving a datatransmission on eigenmodes of the second communication link. The datatransmission is sent at rates selected based on the pilot and thefeedback information.

Various aspects and embodiments of the invention are described in detailbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a scheme for AP-initiated data transmission on eigenmodes.

FIG. 2 shows a process for transmitting data on eigenmodes with lowoverhead.

FIGS. 3 and 4 show two schemes for AP-initiated data transmission oneigenmodes with low overhead.

FIG. 5 shows a scheme for UT-initiated data transmission on eigenmodes.

FIG. 6 shows a scheme for UT-initiated data transmission on eigenmodeswith low overhead.

FIG. 7 shows a process for transmitting data on eigenmodes with lowoverhead.

FIG. 8 shows a block diagram of an access point and a user terminal.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments.

The rate selection techniques described herein may be used for datatransmission on the downlink and uplink. The downlink (or forward link)refers to the communication link from an access point to a userterminal, and the uplink (or reverse link) refers to the communicationlink from the user terminal to the access point. For clarity, much ofthe following description is for downlink data transmission from theaccess point to the user terminal. An access point may also be called abase station, a base transceiver station, and so on. A user terminal mayalso be called a mobile station, a user equipment, a wireless device,and so on.

A downlink MIMO channel formed by T antennas at the access point and Rantennas at the user terminal may be characterized by an R×T channelresponse matrix H, which may be expressed as:

$\begin{matrix}{{\underset{\_}{H} = \begin{bmatrix}h_{1,1} & h_{1,2} & \ldots & h_{1,T} \\h_{2,1} & h_{2,2} & \ldots & h_{2,T} \\\vdots & \vdots & \ddots & \vdots \\h_{R,1} & h_{R,2} & \ldots & h_{R,T}\end{bmatrix}},} & {{Eq}\mspace{14mu} (1)}\end{matrix}$

where entry h_(i,j), for i=1, . . . , R and j=1, . . . , T, is thecoupling or complex gain between AP antenna j and UT antenna i. Forsimplicity, the MIMO channel is assumed to be flat fading, and thecoupling between each pair of AP and UT antennas is represented with asingle complex gain h_(i,j).

The channel response matrix H may be diagonalized to obtain S eigenmodesor orthogonal spatial channels of the downlink MIMO channel. Thisdiagonalization may be achieved by performing either singular valuedecomposition of H or eigenvalue decomposition of a correlation matrixof H, which is R=H^(H)H, where H^(H) denotes the conjugate transpose ofH. For clarity, singular value decomposition is used in the followingdescription. The singular value decomposition of H may be expressed as:

H=U·Σ·V ^(H),  Eq (2)

where

U is an R×R unitary matrix of left eigenvectors of H;

Σ is an R×T diagonal matrix of singular values of H; and

V is a T×T unitary matrix of right eigenvectors of H.

A unitary matrix Q is characterized by the property Q^(H)Q=I, where I isthe identity matrix. The columns of a unitary matrix are orthogonal toone another, and each column has unit power. The right eigenvectors in Vmay be used for spatial processing to transmit data on the eigenmodes ofH. The left eigenvectors in U may be used for receiver spatialprocessing to recover the data transmission sent on the eigenmodes of H.The diagonal matrix Σ contains non-negative real values along thediagonal and zeros elsewhere. These diagonal entries are referred to assingular values of H and represent the channel gains for the eigenmodes.Singular value decomposition is described by Gilbert Strang in “LinearAlgebra and Its Applications,” Second Edition, Academic Press, 1980.

The access point performs spatial processing for eigensteering asfollows:

x=V·s,  Eq (3)

where s is a vector with up to S data symbols to be sent on the Seigenmodes; and

x is a vector with T transmit symbols to be sent from the T AP antennas.

Eigensteering refers to transmission of data on eigenmodes of a MIMOchannel.

As used herein, a “data symbol” is a modulation symbol for data, a“pilot symbol” is a modulation symbol for pilot, a “transmit symbol” isa symbol to be sent from a transmit antenna, a “received symbol” is asymbol obtained from a receive antenna, and a symbol is a complex value.A pilot is a transmission that is known a priori by both thetransmitting and receiving stations. A pilot may also be referred to assounding, training, a reference transmission, a preamble, and so on. Forclarity, the following description assumes that one data stream is senton each eigenmode.

The received symbols at the user terminal may be expressed as:

r=H·x+n=H·V·s+n=H _(eff) ·s+n,  Eq (4)

where

r is a vector with R received symbols from the R UT antennas;

H_(eff)=H·V is an effective MIMO channel response matrix for vector s;and

n is a noise vector.

For simplicity, the noise is assumed to be additive white Gaussian noise(AWGN) with a zero mean vector and a covariance matrix ofφ_(nn)=σ_(noise) ²·I, where σ_(noise) ² is the variance of the noise.The user terminal may recover the transmitted data symbols using variousreceiver spatial processing techniques such as a full-CSI technique, aminimum mean square error (MMSE) technique, and a zero-forcing (ZF)technique.

The user terminal may derive a spatial filter matrix based on thefull-CSI, MMSE, or zero-forcing technique, as follows:

M _(fcsi)=Σ⁻¹ ·U ^(H) =H _(eff) ^(H),  Eq (5)

M _(mmse) =D _(mmse) ·[H _(eff) ^(H) ·H _(eff)+σ_(noise) ² ·I] ⁻¹ ·H_(eff) ^(H),  Eq (6)

M _(:f) =[H _(eff) ^(H) ·H _(eff)]⁻¹ ·H _(eff) ^(H) =R _(eff) ⁻¹ ·H_(eff) ^(H),  Eq (7)

where D_(mmse)=diag {[H_(eff) ^(H)·H_(eff)+σ_(noise) ²·I]⁻¹·H_(eff)^(H)·H_(eff)}⁻¹.

The user terminal may perform receiver spatial processing as follows:

ŝ=M·r=s+ñ,  Eq (8)

where

M is a spatial filter matrix, which may be equal to M_(fcsi), M_(mmse)or M_(:f);

ŝ is a vector with up to S detected data symbols; and

ñ is the noise after the receiver spatial processing.

The detected data symbols in ŝ are estimates of the transmitted datasymbols in s.

The SNR of each eigenmode m, for m=1, . . . , S, may be expressed as:

$\begin{matrix}{{{SNR}_{{fesi},m} = {10\; {\log_{10}\left( \frac{P_{m} \cdot \sigma_{m}^{2}}{\sigma_{noise}^{2}} \right)}}},} & {{Eq}\mspace{14mu} (9)} \\{{{SNR}_{{mmse},m} = {10\; {\log_{10}\left( {\frac{q_{m}}{1 - q_{m}} \cdot P_{m}} \right)}}},} & {{Eq}\mspace{14mu} (10)} \\{{{SNR}_{{zf},m} = {10\; {\log_{10}\left( \frac{P_{m}}{r_{m} \cdot \sigma_{noise}^{2}} \right)}}},} & {{Eq}\mspace{14mu} (11)}\end{matrix}$

where

P_(m) is the transmit power used for eigenmode m;

σ_(m) is the singular value for eigenmode m, which is the m-th diagonalelement of Σ;

q_(m) is the m-th diagonal element of D_(mmse) ⁻; and

r_(m) is the m-th diagonal element of R_(eff) ⁻¹,

SNR_(fcsi,m), SNR_(mmse,m), and SNR_(:f,m) are the SNR of eigenmode mfor the full-CSI, MMSE, and zero-forcing techniques, respectively, andare in units of decibel (dB). The term P_(m)/σ_(noise) ² is oftenreferred to as the received SNR. The terms SNR_(fcsi,m), SNR_(mmse,m),and SNR_(:f,m) are often referred to as the post-detection SNRs, whichare the SNRs after the receiver spatial processing.

The rates for the eigenmodes may be selected based on the SNRs of theseeigenmodes. The rate selection is dependent on the rate selection schemesupported by the system. In one rate selection scheme, the system allowsa rate to be independently selected for each eigenmode based on the SNRof that eigenmode. The system may support a set of rates, and eachsupported rate may be associated with a particular minimum SNR requiredto achieve a specified level of performance, e.g., 1% packet error rate(PER). The required SNR for each supported rate may be obtained bycomputer simulation, empirical measurements, and so on. The set ofsupported rates and their required SNRs may be stored in a look-uptable. The SNR of each eigenmode, SNR_(m), may be compared against therequired SNRs for the supported rates to determine the highest rateR_(m) supported by that SNR_(m). The rate R_(m) selected for eacheigenmode is associated with the highest data rate and a required SNRthat is less than or equal to SNR_(m), or SNR_(req)(R_(m))≦SNR_(m).

In another rate selection scheme, the system allows only certaincombinations of rates to be used for data transmission. The set of ratecombinations allowed by the system is often called a vector-quantizedrate set. A rate combination may also be called a modulation codingscheme (MCS) or some other terminology. Each allowed rate combination isassociated with a specific number of data streams to transmit, aspecific rate for each data stream, and an overall throughput for all ofthe data streams. The SNRs of the eigenmodes may be used to select oneof the allowed rate combinations.

The access point uses the following information to transmit data on theeigenmodes of the downlink MIMO channel:

a set of right eigenvectors in V; and

a set of rates for data streams sent on the eigenmodes.

Different rates may be used for different eigenmodes since theseeigenmodes may achieve different SNRs. The access point may obtain theeigenvectors and the rates for the eigenmodes in various manners.

In a time division duplex (TDD) system, the downlink and uplink sharethe same frequency band, and the downlink and uplink channel responsesmay be assumed to be reciprocal of one another. That is, if H is thechannel response matrix from antenna array X to antenna array Y, then areciprocal channel implies that the coupling from array Y to array X isgiven by H^(T), where H^(T) denotes the transpose of H. However, theresponses of the transmit and receive chains at the access point aretypically different from the responses of the transmit and receivechains at the user terminal. Calibration may be performed to derivecorrection matrices that can account for differences in the responses ofthe transmit/receive chains at the two stations. The application of thecorrection matrices at these two stations allows a calibrated channelresponse for one link to be expressed as a transpose of a calibratedchannel response for the other link. For simplicity, the followingdescription assumes a flat frequency response for the transmit/receivechains. The downlink channel response matrix is H_(dl)=H, and the uplinkchannel response matrix is H_(ul)=H^(T).

The singular value decomposition of H_(dl) and H_(ul) may be expressedas:

H _(dl) =U·Σ·V ^(H) and H _(ul) =V*·Σ ^(T) ·U ^(T),  Eq (12)

where V* is a complex conjugate of V. As shown in equation (12), U and Vare matrices of left and right eigenvectors of H_(dl), and V* and U* arematrices of left and right eigenvectors of H_(ul).

The access point performs spatial processing with V to transmit data oneigenmodes to the user terminal. The user terminal performs receiverspatial processing with U^(H) (or H and V) to recover the downlink datatransmission. One station may transmit an unsteered MIMO pilot that maybe used by the other station to obtain an estimate of H. An unsteeredMIMO pilot is a pilot comprised of N pilot transmissions sent from Nantennas, where the pilot transmission from each antenna is identifiableby the receiving station. N=T for a downlink unsteered MIMO pilot sentby the access point, and N=R for an uplink unsteered MIMO pilot sent bythe user terminal. The transmitting station may orthogonalize the Npilot transmissions in (1) code domain by using a different orthogonalsequence (e.g., Walsh sequence) for each pilot transmission, (2)frequency domain by sending each pilot transmission on a differentfrequency subband, or (3) time domain by sending each pilot transmissionin a different time interval. In any case, the receiving station is ableto obtain an estimate of H based on the unsteered MIMO pilot receivedfrom the transmitting station. For simplicity, the following descriptionassumes no errors in channel estimation.

Singular value decomposition is computationally intensive. Thus, it maybe desirable to have the access point perform singular valuedecomposition of H to obtain the eigenvectors in V. The access point maythen transmit a steered MIMO pilot, which is a pilot sent on theeigenmodes of the MIMO channel. The steered MIMO pilot may be generatedas follows:

x _(pilot,m) =v _(m) ·p _(m),  Eq (13)

where

v_(m) is the right eigenvector for eigenmode m, which is the m-th columnof V;

p_(m) is a pilot symbol transmitted on eigenmode m; and

x_(pilot,m) is a transmit vector for the steered MIMO pilot foreigenmode m.

The access point may transmit a complete steered MIMO pilot on alleigenmodes in one or multiple (consecutive or non-consecutive) symbolperiods.

The received steered MIMO pilot at the user terminal may be expressedas:

r _(pilot,m) =H·x _(pilot,m) +n=U·Σ·V ^(H) ·v _(m) ·p _(m) +n=u_(m)·σ_(m) ·p _(m) +n,  Eq (14)

where r_(pilot,m) is a received vector for the steered MIMO pilot foreigenmode m; and

u_(m) is the left eigenvector for eigenmode m, which is the m-th columnof U.

Equation (14) indicates that the user terminal may obtain (1) anestimate of U, one column at a time, and (2) an estimate of Σ, onesingular value σ_(m) at a time, based on the steered MIMO pilot from theaccess point. The user terminal can obtain estimates of the eigenvectorsand the singular values without having to perform singular valuedecomposition.

The user terminal typically selects the rates for the eigenmodes of thedownlink

MIMO channel and sends the selected rates back to the access point. Theaccess point typically cannot select the rates for the downlink MIMOchannel based solely on an uplink MIMO pilot from the user terminalbecause of various factors such as, e.g., (1) different receiver noiselevels at the access point and the user terminal, (2) differentinterference levels observed by the access point and the user terminal,and/or (3) different transmit powers used for uplink MIMO pilot anddownlink data transmission.

FIG. 1 shows an exemplary pilot and data transmission scheme 100 totransmit data on eigenmodes of the downlink MIMO channel. The timelinefor the access point and the timeline for the user terminal are notnecessarily drawn to scale in FIG. 1.

Initially, the access point sends to the user terminal a request for apilot, which may be called a pilot request (Pilot Req) or a trainingrequest (TRQ) (block 110). The user terminal receives the pilot requestand, in response, transmits an unsteered MIMO pilot in a sounding packet(block 112). The access point receives the unsteered MIMO pilot,estimates the channel response matrix H, and decomposes H to obtaineigenvectors. The access point then transmits a steered MIMO pilot and arequest for rate feedback, which may be called a rate request (Rate Req)or an MCS request (MRQ) (block 114). The user terminal receives thesteered MIMO pilot, estimates the SNR of each eigenmode based on thesteered MIMO pilot, and selects rates for the eigenmodes based on theSNRs of the eigenmodes. The user terminal then sends back the selectedrates for the eigenmodes (block 116). The access point receives theselected rates from the user terminal, processes (e.g., encodes andmodulates) data based on the selected rates, and spatially processes thedata based on the eigenvectors. The access point then transmits asteered MIMO pilot and steered data to the user terminal (block 118).

Transmission scheme 100 allows the access point to transmit data at theproper rates on the eigenmodes of H without the need for the userterminal to perform singular value decomposition. However, four overheadtransmissions are needed for blocks 110 through 116 in order to transmitdata with eigensteering in block 118. The four overhead transmissionsmay wipe out the gain in higher overall throughput achieved witheigensteering. As an example, for a system in which the four overheadtransmissions require 264 microseconds (μs), when the raw data rate witheigensteering is assumed to be 140 Mbps, and the raw data rate withouteigensteering is assumed to be 33% lower, the payload size should exceed8 kilobytes in order to amortize the overhead for eigensteering. Forsmaller payload sizes, performance is better without eigensteeringbecause of lower overhead.

FIG. 2 shows a process 200 for transmitting data on eigenmodes of thedownlink MIMO channel with low overhead. Initially, the access pointsends a request for pilot and feedback information, e.g., TRQ and MRQ(block 212). The access point also transmits a downlink (DL) unsteeredMIMO pilot, e.g., along with the request for pilot and feedbackinformation (block 214).

The user terminal receives and processes the downlink unsteered MIMOpilot and estimates the downlink channel quality, which may bequantified as described below (block 216). The user terminal then sendsfeedback information indicative of the downlink channel quality to theaccess point (block 218). The user terminal also transmits an uplink(UL) unsteered MIMO pilot, e.g., along with the feedback information(block 220).

The access point receives the uplink unsteered MIMO pilot, estimates thechannel response matrix H based on the unsteered MIMO pilot, anddecomposes H to obtain eigenvectors V and singular values for theeigenmodes of H (block 222). The access point also processes the uplinkunsteered MIMO pilot and estimates the uplink channel quality (block224). The access point then estimates the SNRs of the eigenmodes basedon the estimated uplink channel quality, the singular values, and thefeedback information from the user terminal, as described below (block226). The access point selects the rates for the eigenmodes based on theestimated SNRs of the eigenmodes (block 228). The access point thenprocesses (e.g., encodes and modulates) data based on the selected ratesto obtain data symbols (block 230). The access point performs spatialprocessing on the data symbols with the eigenvectors V, e.g., as shownin equation (3), and transmits steered data and a downlink steered MIMOpilot on the eigenmodes to the user terminal (block 232). The accesspoint informs the user terminal of the rates used for the currentdownlink data transmission.

The user terminal receives the downlink steered MIMO pilot and estimatesthe effective channel response matrix H_(eff) (block 234). The userterminal then performs receiver spatial processing on the downlink datatransmission with H_(eff), e.g., as shown in equations (5) through (8)(block 236). The user terminal processes (e.g., demodulates and decodes)the detected data symbols based on the rates selected by the accesspoint to obtain decoded data (block 238).

FIG. 3 shows an improved pilot and data transmission scheme 300 that maybe used for process 200 in FIG. 2. For scheme 300, the access pointsends a request for pilot and feedback information and a downlinkunsteered MIMO pilot in a first overhead transmission (block 310). Thefirst overhead transmission may be, e.g., an Initiator Aggregate Control(IAC) message with the training request (TRQ) and MCS request (MRQ)fields being set (or IAC+TRQ+MRQ). The user terminal sends an uplinkunsteered MIMO pilot and feedback information in a second overheadtransmission (block 312). The second overhead transmission may be, e.g.,a Responder Aggregate Control (RAC) message with an MCS feedback field(MFB) being set and further including a sounding packet (orRAC+MFB+sounding packet). The access point then transmits a steered MIMOpilot and steered data to the user terminal (block 314).

Transmission scheme 300 allows the access point to transmit data at theproper rates on the eigenmodes of H using only two overheadtransmissions. Comparing scheme 300 in FIG. 3 to scheme 100 in FIG. 1,blocks 110 and 114 in FIG. 1 are essentially combined into block 310 inFIG. 3, and blocks 112 and 116 in FIG. 1 are essentially combined intoblock 312 in FIG. 3. The major difference between the two schemes isthat the user terminal sends back (1) the rates for the eigenmodes inblock 116 (since a steered MIMO pilot is available) and (2) feedbackinformation for the downlink MIMO channel in block 312 (since anunsteered MIMO pilot is available). For scheme 300, the access pointperforms additional processing to select the rates for data transmissionon the eigenmodes of the downlink.

In block 214 in FIG. 2 and block 310 in FIG. 3, the access point sends Tpilot transmissions from T AP antennas for the downlink unsteered MIMOpilot. The user terminal may estimate the SNR for each AP antenna basedon the pilot transmission received from that AP antenna. The SNRs forthe T AP antennas are called downlink SNRs and are denoted as SNR_(dl,i)for i=1, . . . , T.

The user terminal may send the feedback information in various forms. Inan embodiment, the feedback information comprises quantized values ofthe downlink SNRs. In another embodiment, the user terminal derives anaverage downlink SNR as follows:

$\begin{matrix}{{SNR}_{dl} = {\frac{1}{T} \cdot {\sum\limits_{i = 1}^{T}{{SNR}_{{dl},i}.}}}} & {{Eq}\mspace{14mu} (15)}\end{matrix}$

The feedback information then comprises a quantized value of SNR_(dl).The downlink SNRs and the average downlink SNR are different forms of anSNR estimate for the downlink.

In yet another embodiment, the user terminal selects a set of ratesbased on the downlink SNRs. The feedback information comprises theselected rates, which may be viewed as coarse quantized values of thedownlink SNRs. In yet another embodiment, the user terminal selects asingle rate based on the average downlink SNR, and the feedbackinformation comprises the selected rate. In yet another embodiment, theuser terminal selects a rate combination based on the downlink SNRs, andthe feedback information comprises the selected rate combination. In yetanother embodiment, the feedback information comprises an overallthroughput for the selected rates or the selected rate combination. Inyet another embodiment, the feedback information comprises a noise flooror noise variance σ_(noise) ² observed at the user terminal.

In yet another embodiment, the feedback information comprisesacknowledgments (ACKs) and/or negative acknowledgments (NAKs) sent bythe user terminal for data packets received from the access point. Theaccess point may maintain a power control loop that adjusts a target SNRfor the user terminal based on the received ACKs/NAKs. The access pointmay use the target SNR to select the appropriate rates for downlinktransmission, as described below.

In general, the feedback information may comprise any type ofinformation that is indicative of the downlink channel quality. Thefeedback information may comprise information sent by one or more layerssuch as a physical layer, a link layer, and so on.

The feedback information may be sent in various manners. In anembodiment, the feedback information is sent in a message having theproper format and fields. This message may be a control message at thelink layer and may be sent whenever there is feedback information tosend. In another embodiment, the feedback information is sent in one ormore designated fields of a frame or packet, e.g., at the physicallayer. The designated fields may be available in each frame or packetthat is transmitted and may be set whenever there is feedbackinformation to send.

The access point may select the rates for the eigenmodes in variousmanners, e.g., depending on the type of feedback information receivedfrom the user terminal. To simplify the rate selection by the accesspoint, the noise and interference at the user terminal may be assumed tobe approximately constant across spatial dimension, and the noise andinterference at the access point may also be assumed to be approximatelyconstant across spatial dimension.

In an embodiment, the SNRs of the eigenmodes are estimated as follows:

SNR _(fcsi,dl,m) =SNR _(fcsi,ul,m)−(SNR _(ul) −SNR _(dl)),  Eq (16)

where

SNR_(dl) is an SNR estimate for the downlink;

SNR_(ul) is an SNR estimate for the uplink;

SNR_(fcsi,ul,m) is the SNR of eigenmode in on the uplink; and

SNR_(fcsi,dl,m) is an estimate of the SNR of eigenmode m on thedownlink.

The SNRs in equation (16) are all in units of dB. The access point mayobtain SNR_(dl) based on the feedback information from the user terminaland may obtain SNR_(ul) based on the uplink unsteered MIMO pilot. Theaccess point may obtain SNR_(fcsi,ul,m) for each eigenmode by (1)decomposing H to obtain the singular values of H and (2) computingSNR_(fcsi,ul,m) for m=1, . . . , S, e.g., as shown in equation (9),where σ_(noise) ² is the noise variance at the access point.

In another embodiment, the SNRs of the eigenmodes are estimated asfollows:

SNR _(fcsi,dl,m) =SNR _(fcsi,ul,m)−(SNR _(ul) −SNR _(dl))−SNR _(bo),  Eq(17)

where SNR_(bo) is a back-off factor used to account for estimationerrors. The back-off factor may be selected based on variousconsiderations such as, e.g., the type of feedback information (e.g.,SNRs or rates) sent by the user terminal, the age of the feedbackinformation, and so on.

In another embodiment, the feedback information comprises one or morerates selected by the user terminal. The access point may convert therates to SNRs and then compute an average downlink SNR based on theconverted SNRs, as shown in equation (15). The access point may then usethe average downlink SNR to estimate the SNRs of the eigenmodes, e.g.,as shown in equation (16) or (17). In yet another embodiment, thefeedback information comprises an overall throughput for the downlink.The access point may convert the overall throughput to an overalldownlink SNR. The access point may also derive an overall uplink SNR andmay use the overall downlink and uplink SNRs to estimate the SNRs of theeigenmodes.

In yet another embodiment, the SNRs of the eigenmodes are estimated asfollows:

SNR _(fcsi,dl,m) =SNR _(fcsi,ul,m) +ASYM(AP,UT),  Eq (18)

where ASYM (AP, UT) is an asymmetric parameter that indicates thedifference in received SNR at the user terminal when the access pointtransmits at a known power level on a known channel to the userterminal. For example, the access point may be equipped with fourantennas, transmit at 17 dBm, and have a noise figure of 6 dB. The userterminal may be equipped with two antennas, transmit at 10 dBm, and havea noise figure of 10 dB. The RSL observed at the user terminal when theaccess point transmits at full power on a lossless channel may becomputed as:

RSL(AP→UT)=17 dBm−10 dB−10 log₁₀(2)=10 dBm.  Eq (19)

The RSL observed at the access point when the user terminal transmits atfull power on a lossless channel may be computed as:

RSL(UT→AP)=14 dBm−6 dB−10 log₁₀(4)=14 dBm.  Eq (20)

The asymmetric parameter ASYM (AP, UT) may then be computed as:

ASYM(AP,UT)=RSL(AP→UT)−RSL(UT→AP)=−4 dBm.  Eq (21)

The asymmetric parameter may also be determined based on the receivedSNRs at the access point and the user terminal, as follows:

ASYM(AP,UT)=SNR _(ut) −SNR _(ap),  Eq (22)

where SNR_(ap) is an SNR estimate for the uplink, and

SNR_(ut) is an SNR estimate for the downlink.

The access point may obtain SNR_(ap) based on an unsteered MIMO pilotreceived from the user terminal. The access point may derive SNR_(ut)based on feedback information (e.g., SNR, rate, ACK/NAK, and so on) sentby the user terminal. For example, SNR^(ut) may be a target SNR that isadjusted based on ACKs/NAKs received from the user terminal.

In general, the SNRs of the eigenmodes may be estimated in variousmanners based on the feedback information and the uplink unsteered MIMOpilot received from the user terminal. The access point selects therates for the eigenmodes based on the SNRs of the eigenmodes. The accesspoint may select a rate for each eigenmode based on the SNR of thateigenmode. The access point may also select a rate combination based onthe SNRs of all eigenmodes.

For scheme 300 in FIG. 3, the user terminal transmits an unsteered MIMOpilot and feedback information in a single overhead transmission. Theuser terminal may also send the pilot and feedback informationseparately.

FIG. 4 shows another pilot and data transmission scheme 400 foreigensteering on the downlink with low overhead. Scheme 400 may be usedfor a scenario in which the access point already has feedbackinformation from the user terminal, e.g., from a prior data transmissionto the user terminal. For scheme 400, the access point sends a requestfor pilot (block 410). The user terminal receives the pilot request and,in response, transmits an uplink unsteered MIMO pilot (block 412). Theaccess point estimates the channel response matrix H based on the uplinkunsteered MIMO pilot, decomposes H to obtain the eigenvectors andsingular values, and derives an uplink SNR estimate based on the uplinkunsteered MIMO pilot. The access point selects the rates for theeigenmodes based on the singular values, the uplink SNR estimate, andthe feedback information already available at the access point. Theaccess point may use an appropriate back-off factor in equation (17) toaccount for the age of the feedback information. For example, aprogressively larger back-off factor may be used for progressively olderfeedback information. The access point then transmits a steered MIMOpilot and steered data to the user terminal (block 414).

Schemes 100, 300 and 400 are for downlink data transmission initiated bythe access point. Data transmission on the downlink may also beinitiated by the user terminal.

FIG. 5 shows a pilot and data transmission scheme 500 for UT-initiatedsteered data transmission on the downlink. For scheme 500, the userterminal sends a request for downlink data transmission and an uplinkunsteered MIMO pilot (block 512). The access point derives eigenvectorsbased on the uplink unsteered MIMO pilot, sends a request for pilot andrate information, and transmits a downlink steered MIMO pilot (block514). The user terminal estimates the SNRs of the eigenmodes based onthe downlink steered MIMO pilot, selects the rates for the eigenmodesbased on the SNRs of the eigenmodes, and sends the selected rates (block516). The access point processes data based on the rates selected by theuser terminal and transmits a downlink steered MIMO pilot and steereddata to the user terminal (block 518). Scheme 500 in FIG. 5 is similarto scheme 100 in FIG. 1, except that the downlink data transmission isinitiated by a data request sent by the user terminal in scheme 500instead of a pilot request sent by the access point in scheme 100.Scheme 500 requires three overhead transmissions to supporteigensteering on the downlink.

FIG. 6 shows an improved pilot and data transmission scheme 600 forUT-initiated steered data transmission on the downlink with lowoverhead. For scheme 600, the user terminal sends a request for downlinkdata transmission, an uplink unsteered MIMO pilot, and possibly feedbackinformation (block 612). The feedback information may comprise (1) thenoise floor or noise variance σ_(noise) ² observed by the user terminal,(2) SNRs of the eigenmodes estimated in a prior downlink datatransmission, or (3) some other information indicative of the downlinkchannel quality. The access point derives eigenvectors and singularvalues based on the uplink unsteered MIMO pilot and selects the ratesfor the eigenmodes based on the uplink unsteered MIMO pilot and thefeedback information. The access point then processes data based on theselected rates and transmits a downlink steered MIMO pilot and steereddata to the user terminal (block 614). Scheme 600 requires a singleoverhead transmission to support eigensteering on the downlink.

The rate selection techniques may be used for downlink data transmissionfrom the access point to the user terminal, as described above. Thesetechniques may also be used for uplink data transmission from the userterminal to the access point and for peer-to-peer data transmission,e.g., from one user terminal to another user terminal. In general, thetransmitting station may be an access point or a user terminal, and thereceiving station may also be an access point or a user terminal. Usingthe techniques described herein, only the transmitting station needs toperform decomposition, and low overhead is required for eigensteering.

FIG. 7 shows a process 700 for transmitting data on eigenmodes with lowoverhead. A pilot (e.g., an unsteered MIMO pilot) is received via afirst communication link (e.g., the uplink) (block 712). Feedbackinformation indicative of the channel quality of a second communicationlink (e.g., the downlink) is also received (block 714). The pilot andfeedback information may be received from a single transmission ormultiple transmissions. The pilot and feedback information may be sentin response to a request for pilot and feedback information, as shown inFIG. 3, or may be sent along with a data request, as shown in FIG. 6.The feedback information may be derived based on a pilot transmitted viathe second communication link, as shown in FIG. 3. In any case, therates for the eigenmodes of the second communication link are selectedbased on the feedback information and the pilot received via the firstcommunication link, e.g., as described above for process 200 in FIG. 2(block 716). Data is processed based on the selected rates andtransmitted on the eigenmodes of the second communication link (block718).

The rate selection techniques described herein may be used forsingle-carrier and multi-carrier MIMO systems. Multiple carriers may beprovided by orthogonal frequency division multiplexing (OFDM) or someother constructs. OFDM effectively partitions the overall systembandwidth into multiple (K) orthogonal subbands, which are also calledtones, subcarriers, bins, and frequency channels. With OFDM, eachsubband is associated with a respective subcarrier that may be modulatedwith data.

For a MIMO system that utilizes OFDM, a channel response matrix H(k) maybe obtained for each subband k and decomposed to obtain the eigenmodesof that subband. The singular values in each diagonal matrix Σ(k), fork=1, . . . , K, may be ordered such that the first column contains thelargest singular value, the second column contains the next largestsingular value, and so on, or σ₁(k)≧σ₂(k)≧ . . . ≧σ_(s)(k), whereσ_(m)(k) is the singular value in the m-th column of Σ(k) after theordering. When the singular values in each matrix Σ(k) are ordered, theeigenvectors (or columns) of the matrix V(k) for that subband are alsoordered correspondingly. A wideband eigenmode may be defined as the setof same-order eigenmode of all K subbands after the ordering, e.g.,wideband eigenmode m includes eigenmode m of all K subbands. Eachwideband eigenmode is associated with a set of K eigenvectors for the Ksubbands. The rate selection may be performed for the S widebandeigenmodes, e.g., similar to that described above for a single-carrierMIMO system.

FIG. 8 shows a block diagram of an access point 810 and a user terminal850. At access point 810, a data/pilot processor 820 receives trafficdata from a data source 812, processes (e.g., encodes, interleaves, andmodulates) the traffic data, and provides data symbols. One data streammay be sent on each cigcnmode, and each data stream may be encoded andmodulated based on the rate selected for that stream/eigenmode.Processor 820 also generates pilot symbols for unsteered and steeredMIMO pilots. A transmit (TX) spatial processor 830 performs spatialprocessing on the data and pilot symbols with eigenvectors and providesT streams of transmit symbols to T transmitter units (TMTR) 832 athrough 832 t. Each transmitter unit 832 conditions a respectivetransmit symbol stream and generates a corresponding modulated signal. Tmodulated signals from transmitter units 832 a through 832 t aretransmitted from T antennas 834 a through 834 t, respectively.

At user terminal 850, R antennas 852 a through 852 r receive themodulated signals transmitted by access point 810, and each antennaprovides a received signal to a respective receiver unit (RCVR) 854.Each receiver unit 854 performs processing complementary to thatperformed by transmitter units 832 and provides received symbols. Areceive (RX) spatial processor 860 performs spatial matched filtering onthe received symbols from all R receiver units 854 based on a spatialfilter matrix and provides detected data symbols. An RX data processor870 processes (e.g., demodulates, deinterleaves, and decodes) thedetected data symbols and provides decoded data.

Controllers 840 and 880 control the operation of various proCessingunits at access point 810 and user terminal 850, respectively. Memoryunits 842 and 882 store data and program codes used by controllers 840and 880, respectively.

For rate selection, a channel processor 878 estimates the downlinkchannel quality and provides a downlink channel quality estimate.Controller 880 provides feedback information indicative of the downlinkchannel quality. The feedback information and pilot symbols for anunsteered MIMO pilot are processed by a data/pilot processor 890 and aTX spatial processor 892 to generate R transmit symbol streams. Rtransmitter units 854 a through 854 r condition the R transmit symbolstreams and generate R modulated signals, which are sent via R antennas852 a through 852 r.

At access point 810, the modulated signals from user terminal 850 arereceived by T antennas 834 and processed by T receiver units 832 toobtain received symbols. The received symbols are further processed byan RX spatial processor 844 and an RX data processor 842 to obtain thefeedback information from user terminal 850. A channel processor 838receives the unsteered MIMO pilot from user terminal 850 and derives achannel estimate for the uplink. The channel estimate may comprise achannel response matrix H and an uplink channel quality estimate.Channel processor 838 decomposes H to obtain eigenvectors and singularvalues for the eigenmodes of H and provides the eigenvectors to TXspatial processor 830. Controller 840 receives the uplink channelquality estimate and the singular values from channel processor 838 andthe feedback information from RX data processor 846, estimates the SNRsof the eigenmodes, selects the rates for the eigenmodes, and providesthe selected rates to TX data processor 820.

The processing to transmit data on eigenmodes of the uplink MIMO channelmay be performed in a manner similar to that described above for thedownlink.

The rate selection techniques described herein may be implemented byvarious means. For example, these techniques may be implemented inhardware, software, or a combination thereof. For a hardwareimplementation, the various units at the access point may be implementedwithin one or more application specific integrated circuits (ASICs),digital signal processors (DSPs), digital signal processing devices(DSPDs), programmable logic devices (PLDs), field programmable gatearrays (FPGAs), processors, controllers, micro-controllers,microprocessors, electronic devices, other electronic units designed toperform the functions described herein, or a combination thereof. Thevarious units at the user terminal may also be implemented within one ormore ASICs, DSPs, processors, and so on.

For a software implementation, the techniques may be implemented withmodules (e.g., procedures, functions, and so on) that perform thefunctions described herein. The software codes may be stored in a memoryunit (e.g., memory unit 842 or 882 in FIG. 8) and executed by aprocessor (e.g., controller 840 or 880). The memory unit may beimplemented within the processor or external to the processor, in whichcase it can be communicatively coupled to the processor via variousmeans as is known in the art.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. An apparatus comprising: a channel processor to receive a first pilotvia a first communication link and to derive a channel estimate for thefirst communication link; and a controller to receive feedbackinformation indicative of channel quality of a second communication linkand to select rates for eigenmodes of the second communication linkbased on the feedback information and the channel estimate.
 2. Theapparatus of claim 1, wherein the controller sends a request for pilotand feedback information, and wherein the first pilot and the feedbackinformation are sent in response to the request.
 3. The apparatus ofclaim 1, further comprising: a pilot processor to generate a secondpilot for transmission via the second communication link, and whereinthe feedback information is derived based on the second pilot.
 4. Theapparatus of claim 1, wherein the channel processor estimates channelquality of the first communication link based on the first pilot, andwherein the controller estimates signal-to-noise-and-interference ratios(SNRs) of the eigenmodes based on the estimated channel quality of thefirst communication link and the feedback information and furtherselects the rates for the eigenmodes based on the SNRs of theeigenmodes.
 5. The apparatus of claim 4, wherein the channel processorobtains a channel response matrix and an SNR estimate for the firstcommunication link based on the first pilot and decomposes the channelresponse matrix to obtain channel gains for the eigenmodes, and whereinthe controller estimates the SNRs of the eigenmodes based on the channelgains for the eigenmodes, the SNR estimate for the first communicationlink, and the feedback information.
 6. The apparatus of claim 4, whereinthe controller selects a rate for each eigenmode based on an SNR of theeigenmode.
 7. The apparatus of claim 4, wherein the controller selects arate combination for the eigenmodes based on the SNRs of the eigenmodes.8. The apparatus of claim 1, wherein the feedback information comprisesa signal-to-noise-and-interference ratio (SNR) estimate for the secondcommunication link.
 9. The apparatus of claim 1, wherein the feedbackinformation comprises at least one rate or an overall throughput for thesecond communication link.
 10. The apparatus of claim 1, wherein thefeedback information comprises acknowledgments or negativeacknowledgments for data packets.
 11. The apparatus of claim 1, whereinthe first pilot and the feedback information are received from a singletransmission sent via the first communication link.
 12. The apparatus ofclaim 1, wherein the feedback information is received for a prior datatransmission sent via the second communication link.
 13. The apparatusof claim 1, further comprising: a data processor to process data basedon the rates selected for the eigenmodes; and a spatial processor tospatially process the data for transmission on the eigenmodes.
 14. Theapparatus of claim 1, wherein the first pilot is an unsteeredmultiple-input multiple-output (MIMO) pilot sent from a first pluralityof antennas and received via a second plurality of antennas.
 15. Amethod of performing rate selection, comprising: receiving a first pilotvia a first communication link; receiving feedback informationindicative of channel quality of a second communication link; andselecting rates for eigenmodes of the second communication link based onthe feedback information and the first pilot.
 16. The method of claim15, further comprising: sending a request for pilot and feedbackinformation, and wherein the first pilot and the feedback informationare sent in response to the request.
 17. The method of claim 15, furthercomprising: transmitting a second pilot via the second communicationlink, and wherein the feedback information is derived based on thesecond pilot.
 18. The method of claim 15, wherein the selecting therates for the eigenmodes comprises estimating channel quality of thefirst communication link based on the first pilot, estimatingsignal-to-noise-and-interference ratios (SNRs) of the eigenmodes basedon the estimated channel quality of the first communication link and thefeedback information, and selecting the rates for the eigenmodes basedon the SNRs of the eigenmodes.
 19. The method of claim 18, wherein theestimating the SNRs of the eigenmodes comprises obtaining a channelresponse matrix for the first communication link based on the firstpilot, decomposing the channel response matrix to obtain channel gainsfor the eigenmodes, and deriving the SNRs of the eigenmodes based on thechannel gains for the eigenmodes, the estimated channel quality of thefirst communication link, and the feedback information.
 20. An apparatuscomprising: means for receiving a first pilot via a first communicationlink; means for receiving feedback information indicative of channelquality of a second communication link; and means for selecting ratesfor eigenmodes of the second communication link based on the feedbackinformation and the first pilot.
 21. The apparatus of claim 20, furthercomprising: means for sending a request for pilot and feedbackinformation, and wherein the first pilot and the feedback informationare sent in response to the request.
 22. The apparatus of claim 20,further comprising: means for transmitting a second pilot via the secondcommunication link, and wherein the feedback information is derivedbased on the second pilot.
 23. The apparatus of claim 20, wherein themeans for selecting the rates for the eigenmodes comprises means forestimating channel quality of the first communication link based on thefirst pilot, means for estimating signal-to-noise-and-interferenceratios (SNRs) of the eigenmodes based on the estimated channel qualityof the first communication link and the feedback information, and meansfor selecting the rates for the eigenmodes based on the SNRs of theeigenmodes.
 24. The apparatus of claim 23, wherein the means forestimating the SNRs of the eigenmodes comprises means for obtaining achannel response matrix for the first communication link based on thefirst pilot, means for decomposing the channel response matrix to obtainchannel gains for the eigenmodes, and means for deriving the SNRs of theeigenmodes based on the channel gains for the eigenmodes, the estimatedchannel quality of the first communication link, and the feedbackinformation.
 25. A method of performing rate selection in amultiple-input multiple-output (MIMO) communication system, comprising:transmitting a first unsteered MIMO pilot via a downlink; receiving asecond unsteered MIMO pilot and feedback information via an uplink,wherein the feedback information is indicative of downlink channelquality estimated based on the first unsteered MIMO pilot; and selectingrates for eigenmodes of the downlink based on the feedback informationand the second unsteered MIMO pilot.
 26. The method of claim 25, whereinthe selecting the rates for the eigenmodes of the downlink comprisesestimating uplink channel quality based on the second unsteered MIMOpilot, obtaining a channel response matrix for the uplink based on thesecond unsteered MI MO pilot, decomposing the channel response matrix toobtain channel gains for the eigenmodes, estimatingsignal-to-noise-and-interference ratios (SNRs) of the eigenmodes basedon the estimated uplink channel quality, the channel gains for theeigenmodes, and the feedback information, and selecting the rates forthe eigenmodes based on the SNRs of the eigenmodes.
 27. An apparatuscomprising: a pilot processor to generate a first pilot for transmissionvia a first communication link; a controller to send feedbackinformation indicative of channel quality of a second communicationlink; and a spatial processor to receive a data transmission oneigenmodes of the second communication link, wherein the datatransmission is sent at rates selected based on the first pilot and thefeedback information.
 28. The apparatus of claim 27, wherein thecontroller receives a request for pilot and feedback information andsends the first pilot and the feedback information in response to therequest.
 29. The apparatus of claim 27, further comprising: a channelprocessor to receive a second pilot via the second communication linkand to derive a signal-to-noise-and-interference ratio (SNR) estimatefor the second communication link based on the second pilot, and whereinthe controller generates the feedback information based on the SNRestimate.
 30. The apparatus of claim 27, wherein the pilot processorgenerates the first pilot as an unsteered multiple-input multiple-output(MIMO) pilot suitable for transmission from a plurality of antennas. 31.A method of performing rate selection, comprising: transmitting a firstpilot via a first communication link; sending feedback informationindicative of channel quality of a second communication link; andreceiving a data transmission on eigenmodes of the second communicationlink, wherein the data transmission is sent at rates selected based onthe first pilot and the feedback information.
 32. The method of claim31, further comprising: receiving a request for pilot and feedbackinformation, and wherein the first pilot and the feedback informationare sent in response to the request.
 33. The method of claim 31, furthercomprising: receiving a second pilot via the second communication link;deriving a signal-to-noise-and-interference ratio (SNR) estimate for thesecond communication link based on the second pilot; and generating thefeedback information based on the SNR estimate.
 34. An apparatuscomprising: means for transmitting a first pilot via a firstcommunication link; means for sending feedback information indicative ofchannel quality of a second communication link; and means for receivinga data transmission on eigenmodes of the second communication link,wherein the data transmission is sent at rates selected based on thefirst pilot and the feedback information.
 35. The apparatus of claim 34,further comprising: means for receiving a request for pilot and feedbackinformation, and wherein the first pilot and the feedback informationare sent in response to the request.
 36. The apparatus of claim 34,further comprising: means for receiving a second pilot via the secondcommunication link; means for deriving asignal-to-noise-and-interference ratio (SNR) estimate for the secondcommunication link based on the second pilot; and means for generatingthe feedback information based on the SNR estimate.